Print substrates, are conveyed within printing machines, by various types of conveying mechanisms. They are carried in this manner to the inking devices and to other devices within the printing machine. The conveyor may be aided by grippers or by suction devices. The conveyor may be carried out with rollers or with conveyor belts. If a print substrate leaves the conveying path provided in the printing machine, it may cause considerable damage within the machine. It may be that at least parts of the print substrate reach the inking device, for example, and cause massive damage therein.
The danger that such defective runs may occur exists in particular with printing machines in which the print substrate is conveyed without grippers. This is the case with digital printing machines, where the print substrate is conveyed via a belt or a roller. The print substrate is held by electrostatic forces or by negative pressure on the belt or on the roller.
It is thus important to prevent the occurrence of defective runs or at least to recognize them in time so that it is still possible to stop the machine or at least prevent it from entering the area endangered by the defective run, before damages occur. This also applies in particular if a sheet-type print substrate is used.
To recognize defective sheets, it is recommended e.g., in EP 0 916 603 A1, to compare the number of sheets fed into the machine with the number of sheets recognized by sensors in the interior of the machine. If the number of sheets in the machine interiors is insufficient, it can be concluded that there are misprints and an alarm is set off and suitable countermeasures are taken. The number of sheets fed in is transmitted by a control signal to the appropriate monitoring devices.
If this device lacks the control signal, e.g., when a machine is restarted, then the recognition of the misprint does not function and it must be switched off for a short period of time during the restart. During this period of time, there is then the danger that unrecognized misprints may cause damages. Naturally, this also applies if other print substrates are used.
In particular, optic sensors are used indirectly, as described in EP 0 916 602 A1, to recognize a defective run. At this point, the print substrate on a conveying mechanism is detected by a change in reflection. By a signal, it is also known at this point when a print substrate should pass the sensor. Thus, if no print substrate is recognized by the sensor at a given point in time, then an alarm is set off and appropriate measures are taken.
Depending on the conveying mechanism and print substrate used, the difference in the reflectivity between print substrate and conveying mechanism may be too small for a normal optic sensor to still be able to detect a difference. This may be caused by dirt on the surface of the conveying mechanism. In this case, a missing print substrate cannot be recognized. For print substrates whose reflectivity approximately matches the reflectivity of the surface of the conveying mechanism, the customary optical sensors are thus relatively unsuitable for recognizing a defective run. At the least, the conveying mechanism must be cleaned regularly.